Stereomicroscopes are used on the one hand to manipulate objects under visual observation and on the other to make fine object details visible. The object manipulation preferably takes place under low magnification and requires good 3D reproduction. For detail recognition rapid switching to high magnifications with high resolution is desired without change of instrument.
Stereomicroscopes provide two views of the object at various observation angles. If the angle between the two observation directions is unusually large, the object appears spatially distorted.
Numerous descriptions of the telescope type of stereomicroscopes appear in the literature: see also “Optical Designs for Stereomicroscopes”, K-P. Zimmer, in International Optical Design Conference 1998, Proceedings of SPIE, Vol. 3482, pages 690-697 (1998) and U.S. Pat. No. 6,816,321. Stereomicroscopes with such a design include—apart from optional bolt-on modules—a main objective, which images the object at infinity, two downstream parallel telescopes for varying the magnification and two observation units comprising a tube lens, inverting system and eyepiece for visual observation with both eyes. The telescopes can be designed as changeable Galilean telescopes with fixed magnification or as a focal zoom systems. According to the prior art two identical telescopes are arranged symmetrically to a plane of symmetry of the device, wherein the plane of symmetry divides the object symmetrically into a right and a left half. The distance between the telescope axes is referred to as the stereo basis. The numerical aperture of this microscope is given by the semi-diameter of the entrance pupil of the telescope divided by the focal length of the main objective.
The numerical aperture of a microscope of this type is limited in the prior art. In order to increase the numerical aperture it is known to expand the entrance pupil diameters and thus the stereo basis which results in the disadvantage of large equipment dimensions, or to shorten the focal length of the main objective, thereby disadvantageously reducing the working distance and increasing the power required of the main objective excessively. In both cases the angle between the observation directions is expanded, resulting in increased spatial distortion.
U.S. Pat. No. 5,603,687 discloses an asymmetrical stereooptic endoscope, in which two objective systems with different diameters of the entrance pupils are arranged parallel next to each other. Both objectives produce images of the object on a sensor surface via light conductors or light fibers. From these CCD sensors for example, the image data are transmitted after digital processing to a monitor, that is to say they can be spacially perceived for example with a stereomonitor. It is stated that despite varying diameters of the two endoscopic channels the viewer perceives a stereoscopic image with a resolution and a brightness, as they result from the channel of larger diameter. The second channel of smaller diameter primarily serves to produce a stereoscopic vision or impression.
The conditions in the case of a stereomicroscope of the telescope type of the design as described above are in principle different than in the case of an endoscope in accordance with U.S. Pat. No. 5,603,687. Firstly, the viewing of the object takes place as a rule (at least also) directly with the eyes, without prior digital processing. Such digital processing will or can be used, if additionally documentation is to be made via connected cameras. It is not clear from the US document mentioned, how in the case of the embodiment disclosed there an object can be viewed directly visually. Furthermore, the projection onto a sensor surface (fixed focus) limits the depth of field of the display since the accommodation capacity of the eyes is out of action.
The magnification of an endoscope depends on the object distance. At high magnifications the object distance is normally minimal. In this case the overlap range of the fields of view of the two objectives being arranged next to one another is minimal. Therefore, stereoscopic viewing, which is only possible in the overlap range, is reduced in this case. At low magnifications however the overlap is large, but the numeric aperture is small, which results in high depth of field. Hence it follows that the image definition or quality of 3D objects only reduces slowly with the distance to the focus plane. This circumstance favours the merging of the two fields into a spatial image, in particular if the object depth is less than the depth of field.
A main component of a stereomicroscope of the type described is the telescope systems (discrete magnification changer or continuous zoom) in the two stereo channels. Telescope systems are not common in endoscopy. In the US document mentioned, therefore, a variation of the display scale or reproduction scale is not discussed.
For stereoscopic viewing the depth of field is important. In contrast to the stereoendoscope described above high power stereomicroscopes of the telescope type advantageously use the accommodation capacity of the eyes. A magnification variation takes place without changing the focusing of the equipment. There is no difference in the object clip between the right and the left field over the whole magnification range. The numeric aperture and thus the resolution of the stereomicroscope are adapted to the magnification and prevent empty magnifications. At high magnifications the depth of field is very small, in many cases smaller than the object depth in such arrangements. The image quality of 3D objects therefore considerably decreases with the distance to the focus plane. Thus, it cannot be assumed that the merging of the fields to a spatial image observed with a stereoendoscope under typically low magnification and high depth of field can be transferred to the conditions, which exist with a high power microscope in particular at high magnifications, if the stereoscopic channels due to different apertures produce images of different resolution and depth of field.
A further, not to be neglected criterion is that of the image brightness, which is different in the case of the US document mentioned, due to the different entrance pupil diameters of the endoscopic channels. Here the digital processing of images has the advantage that both fields can be shown equally brightly on the monitor after corresponding correction. Such corrections are not possible in the case of direct visual viewing, as is the case with stereomicroscopes.
Furthermore, it would be detrimental with an arrangement discussed above, if the higher power of one of the stereoscopic channels could not be used by a user having eyes of different capability, if the stereoscopic channel of higher power was assigned to the eye of lower capability.